This invention relates to a ceramic wiring board and its production process and, more particularly, to a high-density ceramic wiring board having through holes suitable for a hybrid IC or an IC package and its production process.
With the progress in electronic techniques in recent years, enhancement of the density of an electronic device or speed-up of operational function is being advanced. As a result, it becomes necessary that a wiring board has a high density circuit and high reliability and, particularly, a wiring board having characteristics such as low thermal expansion, high thermal conductivity, excellent dimensional stability and excellent electrical properties is required.
Examples of known substrates having characteristics as described above include those comprising ceramics, for example, alumina, beryllia, silicon carbide, and aluminum nitride. Among those ceramic substrates, an alumina substrate is the most widely used as a wiring board for hybrid IC from the viewpoint of characteristics such as mechanical strength, electrical properties, thermal conductivity, thermal expansion, and production cost.
In order to realize a high density or double-sided mounting of the wiring board, it is sometimes necessary that said board be provided with through holes for interconnecting both sides circuits or with through holes for soldering leads of electric components.
However, a substrate comprising a ceramic material such as the above has high hardness and poor processability, so that processing for making holes in a fired ceramic substrate is extremely difficult. Therefore, for a substrate which is used as a ceramic wiring board provided with through holes, a method is usually adopted in which holes are made by punching on the desired positions of a green sheet with a mold and the green sheet is fired. However, this method has problems that it is necessary to fabricate an individual mold for each substrate having a different wiring circuit, so that the lead time is lengthened, the cost is increased, and stock control of fired substrates and molds becomes troublesome. Further it has technical problems that when a substrate has a design in which the arrangement of a plurality of holes is biased, the dimensional accuracy is low because firing shrinkage differs locally.
Therefore, in the development and trial production of a ceramic wiring board having through holes for which the lead time is required to be short, the formation of through holes are carried out by means of, for example, CO.sub.2 gas laser, which is low productivity and very expensive.